If you are speaking in terms of homeowners insurance then basically the replacement cost of your personal property is the cost to replace the items in your home. These would be the contents of the home. Generally replacement cost refers to how much it would cost for these items new.
Your insurance company should be able to provide you with a rate of deprecation depending on the what the property is. Some items will in fact increase in value, jewelry and paintings.
How do I determine the replacement cost for personal property? The replacement cost coverage is the resale value of the property in market when sold. The resale value covers the entire rate of the land, building (with respect to age of building and its belongings) as of the market value.
According to Judge Mathis... Lol... you take the retail cost and immediatly deduct 25%-50% depending on the wear of the item. Jewelery and antiques require an appraiser.